1. Field of the Invention
This invention is concerned with very high capacity transport systems, and in particular with a method of providing a multiplex hierarchy for very high capacity transport systems.
2. Background Art
The dominant signal format in the fiber optic networks follows the synchronous standard SONET in North America and SDH elsewhere. SONET/SDH enables multiplexing, adding/dropping, and general transportation of signals. SONET/SDH is a physical carrier technology, which can provide transport services for ATM, SMDS, frame relay, T1, E1, etc. As well, operation, administration, maintenance and provisioning (OAM&P) features of SONET/SDH provide the ability to reduce the amount of back-to-back multiplexing, and more importantly, network providers can reduce the operation cost of the network.
The SONET standard Bellcore GR-253-CORE and SDH standard ITU G.707 define the physical interface, optical line rates known as optical carrier signals, a frame format, and an OAM&P protocol. Opto/electrical conversion takes place at the periphery of the SONET/SDH network, where the optical signals are converted into a standard electrical format called the synchronous transport signal (STS) in SONET or synchronous transport module (STM) in SDH.
These standards define a basic rate, which is STS-1 for SON ET and STM-1 for SDH. The rate of an STM-1 is three times higher than the rate of an STS-1. Lower and higher rate signals are defined from these basic rates. The lower rate signals are called VT (virtual tributaries) for SONET, and VC (virtual containers) for SDH. The higher rate signals are called STS-N and STM-N, respectively, where “N” takes in practice certain integer values. Examples of SONET STS-N signals are STS-3, STS-12, STS-48, STS-198, etc. Examples of STM-N signals are STM-3, STM-4, STM 64, etc.
Frame structures with a very flexible granularity may be obtained by multiplexing lower rates tributaries in an appropriately sized frame, in a hierarchy that allows correctly delivering the signal to its owner. A tributary may have a phase offset with respect to the beginning of the frame, so that pointers are used to “point” to the first byte of information. The complexity of the frame hierarchical structure increases with the bit-rate, so that assembly and disassembly, as well as processing of the associated pointer information became more complicated. These operations must be performed separately for each signal, which means a large silicon area required with the inherent disadvantages.
For instance, in the case of a STM-1 frame with 64 VC-12 signals, each of the transmitting and receiving nodes must be equipped with common equipment that demultiplexes/multiplexes the VC's and processes the pointers, as well as with 64 independent channels, one for each VC. Each channel needs buffers, means for interpreting the pointers for aligning the incoming tributaries, means for generating the new pointers for the outgoing signals, etc. For same VC-12 granularity, the number of channels for a STM-64 increases to 64×64=4086.
U.S. Pat. No. 5,666,351 (Oksanen et al.) issued on Sep. 9, 1997 and assigned to Nokia Telecommunications Oy discloses a method for assembling and disassembling SDH frame structures with less hardware. At least two signals at the same hierarchy level are processed simultaneously, resulting in a time-division architecture. While the required silicon area is reduced with this architecture, the complexity of pointer processing is not. Also, this patent is not concerned with reducing the bandwidth occupied by the pointers during the transport of hierarchically multiplexed signals.
There remains a need for an efficient multiplexing hierarchy for high capacity transport networks, with a reduced pointer density, for simplifying the operations necessary for pointer processing, and for reducing the bandwidth of the high speed signal.